It is well known that a communication channel often involve nonlinear blocks, i.e. has non-linear transmission characteristics, that generate harmonics and intermodulation. Among intermodulation products, third order intermodulation often is most critical, because it is near in frequency to the useful signals and because from among intermodulation products, it has the most significant amplitude.
An example of an optical communication system 100 comprising such communication channel is schematically illustrated in FIG. 1. The communication channel is between an input port 110 and an output port 120, with a non-linear block 130 in-between over which communication takes place. The non-linear block 130 comprises a laser diode 131 and an optical fiber 132 and a photo diode 133. The laser diode 131 is electrically operated via the input port 110 to emit laser light for communication into and via the optical fiber 132. The photo diode is configured to receive the emitted laser light from the optical fiber and thereby generate an electrical signal at the output port 120.
In order to suppress harmonics and intermodulation in a multi-signal transmission system, a possible solution is careful channel allocation. However, this affects other channels only; intermodulation within the same channel, i.e. where it is generated, will still be an impairment.
Another possible solution is feedback based pre-distortion, e.g. as disclosed in “Adaptive Compensation of the Nonlinear Distortions in Optical Transmitters Using Predistortion”, O. Panagiev, RADIOENGINEERING, VOL. 17, NO. 4, December 2008, pages 55-58. However, complexity and cost related to such solution are relatively high and makes it undesirable to use in many practical situations.
Yet another possible solution is to design an analogue circuits for a specific and measured non-linearity, e.g. as disclosed in “A Predistortion Circuit Design Technique for High Performance Analogue Optical Transmission”, Z. Xu, L. MacEachern, Microsystems and Nanoelectronics Research Conference, 2008 (MNRC 2008), E-ISBN 978-1-4244-5 2921-9, and in “Adaptive CMOS Predistortion Linearizer for Fiber-Optic Links”, R. Sadhwani, B. Jalali, Journal of Lightwave Technology (JLT), VOL. 21, NO. 12, December 2003, pages 213-216. Also this kind of solution is often undesirable in practice as it has to be specifically designed for each specific non-linearity and case, even if caused by the same non-linear components type.
A yet further possible solution may be to design an analogue circuit based on anti-parallel chains of diodes, which chains are placed in series to a non-linear block, or part, such as disclosed in R. B. Childs, V. A. O'Byrne: “Multichannel AM Video Transmission Using a High-Power Nd: YAG Laser and Linearized External Modulator”, IEEE Journal on Selected Areas in Communications, Vol 8, No. 7, September 1990, pp. 1369-1376.